1. Field of the Invention
The present invention relates generally to an electrostatic-magnetic lens for a particle beam apparatus.
2. Description of the Related Art
Particle beam devices are currently being used in all areas of development and manufacture of microelectronic components. The various manufacturing stages of integrated circuits are monitored, masks and wafers are inspected, and micro-structures are generated by electron beam lithography, all with particle beam devices.
Modified scanning electron microscopes that are equipped with retarding field spectrometers and fast beam blanking systems have obtained a particular significance in detecting logic and design errors in large scale integrated circuits, especially during the development phase. For example, the modified scanning electron microscopes are used to mesure the time dependence at selected nodes in the circuit. To avoid charging and/or damaging the radiation sensitive specimens, the electron optical devices are predominantly operated at low accerlating voltages of between 0.5 and 5 kV, so that it is no longer possible to produce high resolution investigations as with conventional scanning electron microscopes.
Therefore, all areas of the semiconductors industry have an increasing need for a high performance, low voltage scanning electron microscope which provides fast and high resolution investigations of microelectronic components.
At low acceleration voltages, the resolution of a scanning electron microscope is determined by the beam diameter d on the specimen, which in turn is essentially defined by the Coulomb repulsion of the electrons, also known as the Boersch effect, which opposes focusing of the beam. Resolution is also hindered by the axial chromatic aberrations of the imaging lenses which increases with the chromatic aberration coefficient C.sub.F and, for a constant width of the energy distribution for the electrons, increases with decreasing primary energy. The beam diameter d is in accordance with the following equation EQU d=(d.sub.O.sup.2 +d.sub.F.sup.2).sup.1/2 ( 1),
where the probe diameter d defines the resolution and d.sub.O denotes the geometrical optical probe diameter expanded by the Coulomb repulsion of the electrons between the beam source and the specimen, in other words, the lateral Boersch effect, and d.sub.F denotes the diameter of the chromatic aberration disk produced by the chromatic aberration, which by the relationship EQU d.sub.F =C.sub.F .multidot..alpha..multidot..DELTA.U/U (2)
is dependent on the beam aperture, on the chromatic aberration coefficient C.sub.F of the lens, on the primary energy eU, and on the width of the energy distribution e.DELTA.U of the electrons. Therefore, it is only possible to improve the resolution by reducing the negative influences of the electron-electron interaction (the energetic Boersch effect which influences the energy width e.DELTA.U and the lateral Boersch effect which influences the probe diameter (d) and the chromatic aberration constant C.sub.F of the lenses used.
In the publication by R. F. W. Pease "Low Voltage Scanning Electron Microscopy", Record of the IEEE 9th Annual Symposium on Electron, Ion and Laser Beam Technology, Berkeley, 9-11 May 1967, pp. 176-187, a scanning electron microscope is disclosed in which primary electrons are initially accelerated to high kinetic energies and are subsequently decelerated to the desired low final energy in an retarding field established immediately above the specimen. By measuring the beam cross section on the specimen, it can be shown that the objective lens of the disclosed arrangement in the retarding mode exhibits significantly smaller chromatic and spherical aberration constants than a magnetic single lens given conventional operation without a retarding field.